1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same, particularly to a semiconductor device for power applications.
2. Description of the Background Art
In a conventional power semiconductor device, particularly a large-current power semiconductor device, an electrode terminal is required to be bonded in a large area to cause large current to flow effectively. Thus, solder has been used for bonding of the electrode terminal (see Japanese Patent Application Laid-Open No. 2006-253516). Meanwhile, in response to increase in the temperature of an environment where the power semiconductor device is used, solder bonding conventionally used has potentially made it impossible to satisfy requested reliability. According to the conventional solder bonding, if the power semiconductor device includes an insulating substrate with a ceramic plate and conductive patterns formed on opposite surfaces of the ceramic plate and a base plate bonded with solder to the insulating substrate, overheating caused during solder bonding of the electrode terminal may remelt the solder used for bonding between the base plate and the insulating substrate. This has made it impossible to use solder types of melting points close to each other to require use of multiple solder types, causing the problem of complicated manufacturing process.
The aforementioned problems may be solved by a method of bonding the electrode terminal ultrasonically to a conductive pattern on the insulating substrate. Ultrasonic bonding is solid-phase bonding and does not require a heating step. Thus, during bonding of the electrode terminal, the electrode terminal can be bonded in a large area without remelting the solder used for bonding between the base plate and the insulating substrate. Additionally, ultrasonic bonding can enhance the reliability of a bonded part compared to solder bonding.
According to ultrasonic bonding, a material to be bonded is bonded by being caused to vibrate ultrasonically while being pressurized through an ultrasonic horn. At this time, the ultrasonic bonding breaks an oxide film on a surface to be bonded to push the oxide film aside to the outside of a bonded part. In response to increase in the area of the bonded part, the oxide film in the center of the bonded part is not pushed aside to the outside of the electrode terminal but it remains in the bonding surface. Thus, a part of the bonding surface with the remaining oxide film is left unbonded. If the oxide film in the center of the terminal can be pushed aside efficiently to the outside of the bonded part, the unbonded part in the center of the terminal can be removed to enhance bonding performance. This can reduce nonuniformity of the quality of the bonded part.
The following technique relating to ultrasonic bonding has been disclosed. A projection is formed on a bonding end surface of an electrode terminal to be bonded to a semiconductor element mounted on an insulating substrate. The projection has a height at least of a level not falling below the thickness of an oxide film formed on a surface of a bonding counterpart. Then, a bonding surface is ultrasonically bonded to a conductive pattern or the semiconductor element.
According to ultrasonic bonding, a material to be bonded is bonded by being caused to vibrate ultrasonically while being pressurized through an ultrasonic horn. An electrode terminal of a semiconductor device is bonded in a large area in one ultrasonic bonding. Thus, an oxide film generated on a surface of the material to be bonded or a contaminant such as dust, oil content or moisture content adhering to the surface of the material to be bonded cannot be pushed aside to the outside of a bonded part during ultrasonic bonding to remain in a bonding surface. A part of the bonding surface with the remaining oxide film or contaminant is left unbonded. For this reason, if the oxide film in the center of the terminal can be pushed aside efficiently to the outside of the bonded part, the unbonded part in the center of the terminal can be removed to enhance bonding performance. This can reduce nonuniformity of the quality of the bonded part while achieving sufficient bonding performance even in a small area.
According to Japanese Patent Application Laid-Open No. 2005-259880, for example, a projection is formed on a bonding end surface of an electrode terminal to be bonded to a semiconductor element mounted on an insulating substrate. The projection has a height at least of a level not falling below the thickness of an oxide film formed on a surface of a bonding counterpart. Then, a bonding surface is ultrasonically bonded to a conductive pattern or the semiconductor element. By using an electrode terminal of this structure, the bonding end surface of the electrode terminal is placed over a bonding surface of a counterpart member and pressure and ultrasonic vibration are applied through an ultrasonic horn. This makes the projection formed in advance on a bonding surface of the electrode terminal slide on a surface of the counterpart member to split and break the oxide film. This forms direct contact with an intrinsic surface under the oxide film. If ultrasonic vibration is applied continuously in this state, plastic flow occurs on the bonding surface of the electrode terminal including the projection to enlarge a part where metal intrinsic surfaces adhere to each other. As a result, the electrode terminal and the counterpart member are ultrasonically bonded with sufficient bonding strength without being affected by the oxide film.
The aforementioned technique of Japanese Patent Application Laid-Open No. 2005-259880 achieves ultrasonic bonding with sufficient bonding strength. However, this technique does not enable a check to see whether the oxide film is removed from a central area of the terminal and resultant ultrasonic bonding has sufficient bonding strength as viewed from an appearance after the bonding. For this reason, if dust is attached to the outside of the projection and a part with the dust is left unbonded, for example, this unbonded part cannot be detected after the bonding.